1. Field of the Art:
The present invention relates to a fixing roller in a fixing unit of an electrophotographic device.
2. Background of the Prior Art:
A known electrophotographic printing device is shown in FIG. 1. During operation, an electrifier 12 charges each point of photosensitive element 14 to a uniform potential. As photosensitive element 14 rotates, certain points are radiated with light 16, such as a laser beam. These radiated points of photosensitive element 14 form an electrostatic latent image, which is then developed into a printed image.
A developer 18 supplies toner to the photosensitive element 14 and the toner adheres to the latent image existing on photosensitive element 14. Transfer unit 20 then transfers the toner on photosensitive element 14 to paper 22.
Once the toner is transferred to paper 22, paper 22 passes through fixing rollers 24, and, through heating and pressurization, the toner will permanently adhere to paper 22. At least one of fixing rollers 24 contains a heating element so that the heating can take place.
FIGS. 2 and 3 further illustrate the construction of a fixing roller 24 that contains a heating element. A fixing roller 24 not containing the heating element is similarly constructed but does not contain the heating element. Disposed within the fixing roller 24 is a heater lamp 34, which heats a cylindrical pressing roll 26 and is detachably mounted to a receptacle disposed in each end of a transmission shaft 28. Pressing roll 26 is typically made from aluminum of high heat conductivity and has a surface coated with a noncohesive resin such as teflon or silicone rubber.
Fixing roller 24 is driven by a rotating transmission shaft 28. Force from transmission shaft 28 is transferred coupling 30, which is connected to the end of shaft 28 as shown in FIG. 2. Coupling 30 contains a drive key 32 that engages a key groove 36 formed at one edge of pressing roll 26. Rotation of transmission shaft 28 therefore causes rotation of fixing roller 24.
However, this coupling arrangement of transmission shaft 28 to fixing roller typically breaks down after about 12,000 pages have been printed, as illustrated in FIG. 6. The cause of this breakdown, as illustrated in FIG. 4, is the presence of a gap L between coupling 30 and pressing roll 26. Gap L is created because of the difference in the thermal expansions of coupling 30 and pressing roll 26 and also the size tolerance.
When a pair of pressing rolls 26 are brought in contact under pressure and driven with drive key 32, each pressing roll 26 exerts pressure on the opposite pressing roll. When pressing rolls 26 rotate, this causes coupling 30 to travel a planetary path within pressing roll 26, as illustrated by the sequence of FIGS. 4A-4C. Specifically, the outer periphery of coupling 30 that is at the pressure point P will always be in the strongest contact with the inside of pressing roll 26.
This motion causes sliding abrasion between drive key 32 and key groove 36 and the creation of abrasion dust. Thus, both drive key 32 and key groove 36 become worn, and, eventually, pressing roll 26 will not rotate. The whole fixing roll 24 must then be replaced, despite the remaining life of the noncohesive resin disposed on pressing roll 26. Also, this motion causes increased external noise, which results in loud and annoying operation.
The above description illustrates that an improved fixing roller is needed to obtain a more reliable and longer lasting electrophotographic device.